linux-kernel - Re: [RFC PATCH 0/6] Add platform device SVM support for ARM SMMUv3

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Date:   Thu, 7 Sep 2017 09:41:42 +0800
From:   Bob Liu <liubo95@...wei.com>
To:     Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Yisheng Xie <xieyisheng1@...wei.com>
CC:     <joro@...tes.org>, <robh+dt@...nel.org>, <mark.rutland@....com>,
        <lorenzo.pieralisi@....com>, <hanjun.guo@...aro.org>,
        <sudeep.holla@....com>, <rjw@...ysocki.net>, <lenb@...nel.org>,
        <will.deacon@....com>, <robin.murphy@....com>,
        <robert.moore@...el.com>, <lv.zheng@...el.com>,
        <iommu@...ts.linux-foundation.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devel@...ica.org>,
        <chenjiankang1@...wei.com>, <xieyisheng@...wei.com>
Subject: Re: [RFC PATCH 0/6] Add platform device SVM support for ARM SMMUv3

On 2017/9/6 17:57, Jean-Philippe Brucker wrote:
> On 06/09/17 02:02, Bob Liu wrote:
>> On 2017/9/5 20:56, Jean-Philippe Brucker wrote:
>>> On 31/08/17 09:20, Yisheng Xie wrote:
>>>> Jean-Philippe has post a patchset for Adding PCIe SVM support to ARM SMMUv3:
>>>> https://www.spinics.net/lists/arm-kernel/msg565155.html
>>>>
>>>> But for some platform devices(aka on-chip integrated devices), there is also
>>>> SVM requirement, which works based on the SMMU stall mode.
>>>> Jean-Philippe has prepared a prototype patchset to support it:
>>>> git://linux-arm.org/linux-jpb.git svm/stall
>>>
>>> Only meant for testing at that point, and unfit even for an RFC.
>>>
>>
>> Sorry for the misunderstanding.
>> The PRI mode patches is in RFC even no hardware for testing, so I thought it's fine for "Stall mode" patches sent as RFC.
>> We have tested the Stall mode on our platform.
>> Anyway, I should confirm with you in advance.
>>
>> Btw, Would you consider the "stall mode" upstream at first? Since there is no hardware for testing the PRI mode.
>> (We can provide you the hardware which support SMMU stall mode if necessary.)
> 
> Yes. What's blocking the ATS, PRI and PASID patches at the moment is the
> lack of endpoints for testing. There has been lots of discussion on the
> API side since my first RFC and I'd like to resubmit the API changes soon.
> It is the same API for ATS+PRI+PASID and SSID+Stall, so the backend
> doesn't matter.
> 

Indeed!

> I'm considering upstreaming SSID+Stall first if it can be tested on
> hardware (having direct access to it would certainly speed things up).

Glad to hear that.

> This would require some work in moving the PCI bits at the end of the
> series. I can reserve some time in the coming months to do it, but I need
> to know what to focus on. Are you able to test SSID as well?
> 

Yes, but the difficulty is our devices are on-chip integrated hardware accelerators which requires complicate driver.
You may need much time to understand the driver.
That's the same case as intel/amd SVM, the current user is their GPU :-(

Btw, what kind of device/method do you think is ideal for testing arm-SVM?

>>>> We tested this patchset with some fixes on a on-chip integrated device. The
>>>> basic function is ok, so I just send them out for review, although this
>>>> patchset heavily depends on the former patchset (PCIe SVM support for ARM
>>>> SMMUv3), which is still under discussion.
>>>>
>>>> Patch Overview:
>>>> *1 to 3 prepare for device tree or acpi get the device stall ability and pasid bits
>>>> *4 is to realise the SVM function for platform device
>>>> *5 is fix a bug when test SVM function while SMMU donnot support this feature
>>>> *6 avoid ILLEGAL setting of STE and CD entry about stall
>>>>
>>>> Acctually here, I also have a question about SVM on SMMUv3:
>>>>
>>>> 1. Why the SVM feature on SMMUv3 depends on BTM feature? when bind a task to device,
>>>>    it will register a mmu_notify. Therefore, when a page range is invalid, we can
>>>>    send TLBI or ATC invalid without BTM?
>>>
>>> We could, but the end goal for SVM is to perfectly mirror the CPU page
>>> tables. So for platform SVM we would like to get rid of MMU notifiers
>>> entirely.
>>>
>>>> 2. According to ACPI IORT spec, named component specific data has a node flags field
>>>>    whoes bit0 is for Stall support. However, it do not have any field for pasid bit.
>>>>    Can we use other 5 bits[5:1] for pasid bit numbers, so we can have 32 pasid bit for
>>>>    a single platform device which should be enough, because SMMU only support 20 bit pasid
>>>>
>>
>> Any comment on this?
>> The ACPI IORT spec may need be updated?
> 
> I suppose that the Named Component Node could be used for SSID and stall
> capability bits. Can't the ACPI namespace entries be extended to host
> these capabilities in a more generic way? Platforms with different IOMMUs
> might also need this information some day.
> 

Hmm, that would be better.
But in anyway, it depends on the ACPI IORT Spec would be extended in next version.

--
Thanks,
Bob Liu